Apparatus for gas-assisted, solid-to-solid thermal transfer with a semiconductor wafer

ABSTRACT

Apparatus and method are provided for effecting gas-assisted, solid-to-solid thermal transfer with a semiconductor wafer. A semiconductor wafer is loaded at its periphery onto a shaped platen. Sufficient contact pressure from the loading is produced between the wafer and the platen so that significant gas pressure may be accommodated against the back side of the wafer without having the wafer bow outwardly or break. Gas under pressure is introduced into the microscopic void region between the platen and the wafer. The gas fills the microscopic voids between the platen and semiconductor wafer. The gas pressure approaches that of the preloading contact pressure without any appreciable increase in the wafer-to-platen spacing. Since the gas pressure is significantly increased without any increase in the wafer-to-platen gap, the thermal resistance is reduced and solid-to-solid thermal transfer with gas assistance produces optimum results.

This invention relates to apparatus for effecting thermal transfer witha semiconductor wafer and, more particularly, relates to apparatus foreffecting solid-to-solid thermal transfer with a semiconductor waferwhich introduces a gas into microscopic voids between the wafer and anunderlying platen to assist in thermal conduction.

In the processing of semiconductor wafers, e.g., in order to fabricateintegrated circuits, it sometimes occurs that wafers are subjected toelevated temperatures. For the diffusion of impurities, the growth ofepitaxial layers, the application of high quality metal films orannealing of metal semiconductor contacts, and the like, such elevatedtemperatures are desirable. In these situations, it is desirable toapply thermal energy in a controlled and uniform manner. For otherapplications, such as ion implantation and etching, thermal energy is anunwanted byproduct. In these latter applications, it may be undesirableto allow the wafers to remain at elevated temperatures since, forexample, gratuitous diffusion beyond prescribed limits, as well as thesegregation of impurities at epitaxial interfaces is not desired. Also,photoresist layers may be affected at these elevated temperatures. Thisproblem is enhanced in the fabrication of large scale integration (LSI)and very large scale integration (VLSI) devices since a large number ofprocessing steps must be used in sequence; in particular, near the endof the processing sequence there are large numbers of doped regions,conducting layers or insulating layers in place and it is not desirableto disturb these physical features by thermal treatment. In thesesituations, one wants to cool the semiconductor wafers in a controlledand uniform manner. Thus, it is desired to raise semiconductor wafers toelevated temperatures when a process step positively requires it and, onthe contrary, to cool a semiconductor wafer to prevent elevatedtemperatures from being attained when unwanted heat is generated.

Previous approaches to effecting thermal transfer with a semiconductorwafer have involved radiation, convective and conductive means. Wafershave been heated by infrared radiation of the exposed upper surface andwafers have been allowed to cool off radiatively. Wafers have beenraised to elevated temperatures by streams of heated gas. And wafershave been heated inductively as they rest on susceptors. Also, wafershave been kept cool by scanning intermittently either or both the ionbeam or the wafer (thereby limiting throughput), providing an activelycooled metal plate, coated with grease or oil, for the semiconductorwafer to rest upon, or applying an electrostatic force to hold a waferagainst a slightly compressible surface on an actively cooled plate.See, e.g., L. D. Bollinger, "Ion Milling for Semiconductor ProductionProcesses", Solid State Technology, November 1977. These prior arttechniques and devices have proven to not be wholly effective at coolingsemiconductor wafers when high ion fluxes or energy levels areexperienced. A convexly curved platen to which a semiconductor wafer isclamped is disclosed in R. A. Faretra, U.S. Pat. No. 4,282,924,"Apparatus for Mechanically Clamping Semiconductor Wafer Against PliableThermally Conductive Surface". The cooling efficiency of this apparatusis limited by the extent to which the backside of the wafer actuallycontacts the thermally conductive surface since, at the microscopiclevel, only small areas of the two surfaces (typically less than 5%)actually come in contact.

The technique of gas conduction is known to permit thermal couplingbetween two opposed surfaces. The technique has been widely employed.For example, in O. E. Andrus, "Multi-Layer Vessel Having a Heat TransferMaterial Disposed Between Layers", U.S. Pat. No. 3,062,507, a gas (orliquid) is placed between layers of a vessel to obtain optimum heattransfer. Gas conduction heat transfer to produce switching in cryogenicpumps is disclosed, for example, in B. S. Denhoy, U.S. Pat. No.3,525,229, "On-Off Thermal Switch for a Cryopump"; T. P. Hosmer, U.S.Pat. No. 3,717,201, "Cryogenic Thermal Switch"; R. W. Stuart, et al.,"Thermal Switch for Cryogenic Apparatus", U.S. Pat. No. 3,430,455; andW. H. Higa, U.S. Pat. No. 3,421,331, "Refrigeration Apparatus". In eachcase, thermal transfer between opposed surfaces is obtained by gasconduction.

In R. V. Stuart, "Cooling Apparatus for Vacuum Chamber", U.S. Pat. No.3,566,960, the problem of inadequate contact between solid surfaces isdiscussed (see column 3, line 2 et. seq.) and a circulating gaseous orliquid medium to cool the workpiece in the vacuum chamber is disclosed.In the same vein, gas conduction cooling of a workpiece, preferably asemiconductor wafer, in a vacuum is shown in M. King and P. H. Rose,"Experiments on Gas Cooling of Wafers", Proceedings, Third InternationalConference on Ion Implantation Equipment and Techniques, QueensUniversity, Kingston, Ontario (May, 1980), and in M. King, U.S. Pat. No.4,264,762, "Method of Conducting Heat to or From an Article BeingTreated Under Vacuum". In this apparatus, gas is introduced into themiddle of a cavity behind a semiconductor wafer. Thermal couplingbetween a support plate and the wafer is achieved through a gas astypically accomplished in the gas conduction art. In practice, however,there is a finite leakage rate due to imperfect seals so that a pressuregradient exists between the middle of the cavity and the periphery.Since the heat conductivity in a gas is proportional to pressure, moreheat is transferred at the center where a higher pressure exists and atemperature gradient exists across the wafer. For certain processes,such as metal coating, this temperature gradient leads to non-uniformprocessing which may be undesirable. In addition, since the wafer is notpressed against a platen, it is free to move as a membrane wheneverappreciable pressure is introduced into the gap between the supportplate and the wafer. Outward movement of the wafer-membrane increasesthe gap so that thermal conduction is decreased, thereby yielding up anygain in conduction due to the increase in gas pressure.

It is therefore an object of the present invention to provide anapparatus and method for effecting gas-assisted, solid-to-solid thermaltransfer with a semiconductor wafer.

It is a further object of the present invention to provide an apparatusand method which combines the benefits of solid contact thermal transferwith the benefits of thermal transfer by gas conduction.

It is another object of the present invention to provide an apparatusand method for effecting gas-assisted, solid-to-solid thermal transferwith a semiconductor wafer wherein the wafer is preloaded to a shapedplaten to permit high gas pressures to be employed without bowing of thewafer and thereby optimizing the gas-assisted component of thermaltransfer.

BRIEF DESCRIPTION OF THE DRAWINGS

For a complete understanding of the apparatus of the present inventionreference may be had to the accompanying figures which are incorporatedherein by reference and in which:

FIG. 1 is an illustration of apparatus of the prior art for solidcontact thermal transfer with semiconductor wafers;

FIG. 2 is an illustration of apparatus of the prior art for gasconduction cooling;

FIG. 3 is a cross sectional view of apparatus of the present invention;

FIG. 4 is a plan view of the apparatus of FIG. 3;

FIG. 5 is a pictorial curve showing thermal transfer in various gaspressure regions;

FIG. 6 is a curve showing centerpoint deflection for a silicon wafer asa function of gas pressure;

FIG. 7 is a curve showing the thermal transfer capacity of nitrogen gasas a function of gas pressure for a typical semiconductor wafer which isallowed to deflect under the influence of gas pressure;

FIG. 8 is a curve showing thermal transfer with the apparatus of thepresent invention as a function of gas pressure; and

FIG. 9 is a curve showing the contributions to net thermal conductionwith a semiconductor wafer made by the solid-to-solid thermal transfercomponent and the gas conduction component.

SUMMARY OF THE INVENTION

Apparatus and method are provided for effecting gas-assisted,solid-to-solid thermal transfer with a semiconductor wafer. Asemiconductor wafer is loaded at its periphery onto a shaped platen.Sufficient contact pressure from the loading is produced across thewafer so that gas pressure up to the magnitude of the initial loadingmay be applied against the back side of the wafer without having thewafer lift off the platen. Gas under significant pressure is introducedinto the microscopic voids between the wafer and the platen while thegap remains nearly constant. Since the gap remains narrow, even at highpressures up to the preloading level, thermal resistance is reduced andthermal transfer is enhanced.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The conduction of heat from one solid to another is a fundamentalthermal transfer phenomenon. The terminology employed depends uponconvention, but such conduction includes transfer in either direction,i.e., includes both heating and cooling. See, e.g., H. Grober, et. al.,Fundamentals of Heat Transfer, Part I, "Conduction of Heat in Solids"(1961). Ideally, the two surfaces of the respective solids which makecontact are in complete contact over all their area. In practicablesystems, however, such as a semiconductor wafer pressed against thesurface of a platen, there are irregularities in the two surfaces on amicroscopic scale. As a consequence, in the field of semiconductorproduction equipment even though a semiconductor wafer is firmly pressedagainst a platen, the actual area in contact on a microscopic scale issubstantially less than ten percent (10%) of the total surface area.This renders solid-to-solid heat transfer less than optimum, especiallyfor semiconductor wafers which are typically processed under vacuumconditions where convective or conductive contributions to transfer donot occur. The factors which determine the effectiveness ofsolid-to-solid thermal transfer in a vacuum are described in M. G.Cooper, et.al., "Thermal Contact Conductance", Int. J. Heat MassTransfer, v. 12, p. 279 (1969).

The technique of gas conduction as applied to semiconductor wafers isexemplified by the apparatus and process of M. King and P. H. Rose,"Experiments on Gas Cooling of Wafers", Proceedings, Third InteractionalConference on Ion Implantation Equipment and Techniques, QueensUniversity, Kingston, Ontario (May 1980), and in M. King, "Method ofConducting Heat to or From an Article Being Treated Under Vacuum", U.S.Pat. No. 4,264,762. A semiconductor wafer is placed above a supportplate with a gap inbetween into which is introduced a gas. Heat isconducted through the gas between the wafer and the support plate. Thepressure of the gas is necessarily below that which would eject thewafer from the support plate, thereby defeating the underlying purposeof thermal transfer. Even if the wafer is firmly clamped to the supportplate the maximum gas pressure allowed is that pressure at which thewafer, as a thin membrane, would begin to deform substantially away fromthe support plate. As shown in FIG. 6, this deformation in a 100 mmwafer becomes considerable at pressure of 1 Torr. Such deformation wouldtherefore be intolerable as thermal transfer would be severely degradedas the gap between the support plate and wafer increased. This is seenby curve `a` in FIG. 7 where thermal transfer is plotted as a functionof pressure for the deformed wafer of FIG. 6. As the wafer starts todeform in the center thermal conductivity for the wafer is quicklydegraded. Thus, the thermal transfer capability of a pure gas conductiontechnique is limited to that which can be attained at pressures belowabout 2 Torr.

In the embodiment of the apparatus of the present invention shown incross-sectional view of FIG. 3 and in plan view in FIG. 4, gas isintroduced through conduit 44 into annular indentation 37 whichcircumscribes the surface of shaped platen 36. Annular indentation 37introduces gas around the periphery of the wafer adjacent the locationwhere the wafer is clamped on the top side and sealed from below by item45. The contact pressure between the surface of shaped platen 36 and theback of wafer 41 will be produced by the application of clamping forcefrom means 42. This pressure is selected to be below that which wouldfracture wafer 41, yet is significant enough to permit accommodation ofsignificant gas pressure behind the wafer. As gas is introduced underpressure from source 43, through valve 40 and channel 44 to annularchannel 37, it fills the microscopic voids in the interface between thebottom of wafer 41 and the surface of platen 36. As gas is introduced aportion of the pressure which holds the wafer in its curved contour isnow provided by the gas. As the pressure is increased, still more of theforce applied to the wafer is contributed by the gas within themicroscopic voids and less of the force is contributed by solid contactof the wafer with the surface of the platen. Even with increased gaspressure the wafer remains in place and until gas pressure equals thepreload pressure. Then, the wafer ceases to be solidly supported by thewafer, and the wafer is lifted off the peaks of the solid surface. Thewafer begins to act as a membrane which is subject to bowing by theapplied gas. Basically, the gas pressure compensates for the forceapplied by means 42 and the wafer floats above the surface of theplaten. Any increase in pressure above this level will be comparable toapplying like increases to a wafer that is not preloaded. Thus, as shownby FIG. 6, under the application of even 1-2 Torr over pressure, thewafer will begin to deform and the thermal conductivity will severelydegrade. This is indicated by portion `c` of FIG. 8 where net thermalconduction by the apparatus of the present invention drops off rapidlywhen the pressure exceeds the preload contact pressure.

The ultimate determination of the thermal transfer capabilities ofplaten 35 is based on the thermal capacity of the heated or cooled fluidwhich circulates through channels 38. The thermal mass of platen 35 issufficiently great so that platen 35 appears as a large heat source orsink to the wafer 41 (typically the wafer has a mass of about 4 grams).

The apparatus and method of the present invention combines the benefitsof solid-to-solid thermal transfer with thermal transfer assistance fromgas conduction. The assistance from gas conduction may be significantsince very significant gas pressures may be attained. As shown in FIG.8, for a preloading pressure of 35 Torr, the pressure behind the wafermay reach nearly 35 Torr without lifting of the wafer. The apparatus ofthe present invention produces solid-to-solid contact between thesemiconductor wafer and a platen by preloading the wafer to the platento achieve significant, preferably uniform, contact pressure across thewafer. Preferably, the means 42 for preloading the wafer 41 applies apreloading force to permit the wafer 41 to accommodate gas pressureacross its back side in the range of 5 Torr to 100 Torr without liftingor bowing of the wafer. Typically, this is on the order of 30-50 Torrbut may be lower or higher. The upper limit of such preloading would bethe pressure at which some wafers would fracture. From external visualinspection one would observe the wafer to be closely clamped onto theplaten. However, on a microscopic level the area in contact is stillsignificantly less than ten percent (10%) of the total area available.This is the case whether the platen has a metallic or resilientpolymeric surface. Into these microscopic voids gas is introduced underpressure. The pressure may be increased up to the level of thepreloading since the pressure of the gas in the voids substitutes forthe lifting force provided by the peaks on the surface of the platen.Essentially gas pressure is increased while the platen-to-wafer gapremains nearly constant, thereby allowing significant gas pressure to beobtained without bowing or lifting the wafer.

To fully appreciate the hybrid character of the present invention(solid-to-solid thermal transfer in conjunction with gas conduction), itis useful to examine the mechanism of thermal transfer by gasconduction. As seen in FIG. 5, at low pressures the rate of thermaltransfer by gas conduction increases linearly with pressure. Here, thedensity of the gas increases with increasing pressure and the mean freepath remains long enough so that the preponderance of gas collisionstake place with either the semiconductor wafer or the platen.Essentially, the gas molecules travel back and forth between the waferand the platen. This pressure condition is called molecular flow. Inthis region, the higher the pressure, the higher the rate of heattransfer between the semiconductor wafer and the platen. For most gasesof interest in a semiconductor production environment, the molecularflow region ranges below about 1 Torr. At sufficiently high pressures orat sufficiently large spacings between the semiconductor wafer and theplaten, the preponderance of the gas collisions begin to occur betweengas molecules rather than with the semiconductor wafer or with theplaten. This condition is described as the laminar flow region.Inbetween the molecular flow and laminar flow regions, there is atransitional region where characteristics of both regions are present.The region where some laminar flow characteristics are present liesabove about 5 Torr for most gases of interest in the semiconductorproduction environment. In this region the gas begins to behave at leastin part like a fluid where thermal conductivity is independent ofpressure. Thus, once this condition is achieved, no benefit is obtainedby increasing the pressure for a given gap. Thermal resistance isreduced only by reducing the gap between the semiconductor wafer and theplaten. The transition from molecular to laminar flow is gradual andwill occur at different points depending upon the platen-to-wafer gap inthe system.

In the laminar flow region the resistance to thermal conductivity ispressure independent and gap dependent. This dependence is seen in FIG.7. The laminar flow region or the transitional region with components oflaminar flow produces the horizontal curves where thermal transfercapacity is independent of pressure. Here, the transitional region (seeFIG. 5) is folded into the pure laminar flow region. When thetransitional or pure laminar flow regions are reached, the thermaltransfer capacity becomes constant for a given spacing between thesemiconductor wafer and platen. This relationship is well established inthe thermal transfer field. See, e.g., H. Grober, et. al., Fundamentalsof Heat Transfer, Chapter 7, `Conduction in Rarefied Gases`, p. 150 et.seq. (1961) and S. Dushman, Scientific Foundations of Vacuum Technique,2nd Ed., p. 43 (1962). The apparatus and method of the present inventionoperates in this pressure condition where the laminar flow is present.To obtain operation under this condition, the semiconductor wafer isloaded at its periphery with a peripheral clamping force as shownschematically in FIG. 3. This clamping is similar to the prior artclamping fixture 10 of FIG. 1 but serves an additional purpose. In theprior art, wafer 13 is held against a convexly curved platen byperipheral clamping ring 36 in order to obtain good solid-to-solidcontact over the area of the wafer. With the present inventionsolid-to-solid contact is obtained and in addition, the loading producesa narrow platen-to-wafer gap on the average so that the assistance fromgas conduction is optimized. Thus, the solid-to-solid thermal transfercomponent is present as well as the gas-assisted component. Thecombination of these components is shown in FIG. 9 where curve `a` givesthe conduction component due to platen-to-wafer contact, curve `b` givesthe contribution due to gas conduction and curve `c` gives the netconduction. As the pressure is increased up to the preload pressure, thesolid contact component is reduced until the wafer lifts off or losescontact with the wafer. The gas conduction component, curve `b`,increases with pressure until the pure laminar flow region is reachedand then is essentially constant with pressure.

With the apparatus of the present invention, it has been found thatpreloading contact pressures of 35 Torr or higher are readily achievableso that corresponding gas pressures may be obtained while retaining thenarrow gap. These pressures are easily high enough to establish somelaminar flow characteristics (see FIG. 5). At the same time that loadingpermits sufficiently high gas pressures, the loading minimizes thespacing between the semiconductor wafer and the platen, therebyheightening the thermal transfer capacity of the gas within themicroscopic voids between the platen and the semiconductor wafer.Generally, the platen has a convex shape. It preferably has a smoothlyfinished metal surface, e.g., of soft aluminum. It has been found thatsuch a solid metal surface produces the preferred solid-to-solid thermalcontact, i.e. produces better than a resilient polymer coating. Thequality of the thermal contact is directly proportional to theconductivity of the metal, inversely proportional to the hardness and isproportional to the frequency of the peaks which rise above theasperities in the surface. See the discussion in M. G. Cooper, et.al.,"Thermal Contact Conductance", Int. J. Heat Mass Transfer, v. 12, p. 279(1969). Pliable, thermally conductive polymers may be used, butgenerally do not possess as high a Figure of Merit. In a preferredembodiment, the curvature of the platen is selected to produce a uniformcontact pressure across the wafer when the wafer is preloaded. Thispreferred curvature is described in the copending application of S.Holden, "Optimum Surface Contour for Conductive Heat Transfer From aThin Flexible Workpiece", Ser. No. 381,668 filed on even date herewith.

The operation of the apparatus of the present invention as contrasted tothe pure gas conduction thermal transfer of the prior art may be seen byreference to FIG. 8. The conventional gas conduction cooling apparatusof the prior art, shown in FIG. 2, has a finite gap 21 between wafer 20and support plate 23. Since there is no preloading to a shaped platen,at a few Torr the wafer begins to deform thereby increasing the gap. Thethermal conductivity drops off sharply as shown by curve `b`. Incontrast, the thermal conductivity for the apparatus of the presentinvention rises as shown in curve `a` until the laminar flow region isreached. When the preload pressure of 35 Torr is exceeded the waferbegins to deform and here, too, thermal conductivity drops off sharplyas shown in curve `c`.

The performance achievable by the method of the present invention wasdemonstrated by implanting a 3-inch photoresist coated silicon waferwith a 2 mA As+ ion beam at 180 KeV. The silicon wafer was clamped tothe apparatus of the present invention. Air was introduced between theplaten and the wafer at a pressure of less than 30 Torr. A surface areaof 51 cm² was implanted. The incident power density was more than 6watts/cm². No deterioration of the photoresist was observed over theentire surface of the wafer.

What is claimed is:
 1. Apparatus for effecting thermal transfer with athin flexible disk in vacuum, comprising:a platen having a nonplanarsurface for receiving said disk and forming a solid-to-solid contactwith the back side of said disk; means for clamping said disk at itsperiphery against said platen surface to produce a platen-to-diskpreloading contact pressure; and means for introducing a gas intomicroscopic voids between said disk and said platen surface at apredetermined pressure above the level sufficient to cause bowing andreduced thermal transfer with a disk which is not preloaded, saidpredetermined gas pressure being less than said preloading contactpressure.
 2. Apparatus as defined in claim 1 wherein said platen surfaceis convexly curved.
 3. Apparatus as defined in claim 2 wherein saidmeans for introducing a gas comprises an open circular channel in saidplaten surface having a radius less than said disk and a conduit in saidplaten for coupling said circular channel to a gas source.
 4. Apparatusas defined in claim 3 further including sealing means in said platensurface radially outward from said circular channel for preventingescape of said gas.
 5. Apparatus as defined in claim 4 wherein saidplaten includes channels for circulation of a heat transfer fluid. 6.Apparatus as defined in claim 1 wherein said platen surface has acontour which produces uniform preloading over the surface area of saiddisk.
 7. Apparatus as defined in claim 6 wherein said disk comprises asemiconductor wafer.
 8. Apparatus as defined in claim 7 wherein saidmeans for clamping said wafer applies a preloading force to permit saidwafer to accommodate gas pressure across its back side in the range of 5Torr to 100 Torr without lifting or bowing to the wafer.
 9. Apparatusfor effecting thermal transfer with a semiconductor wafer in a vacuumprocessing chamber, comprising:a platen having a nonplanar surface forreceiving said wafer and forming a solid-to-solid contact with the backside of said wafer; means for loading said wafer at its peripheryagainst said platen surface so as to produce a platen-to-waferpreloading contact pressure over the surface area of said wafersufficient to maintain solid-to-solid contact between said wafer andsaid platen when gas is introduced into the microscopic voids betweenthe platen and the wafer; and means for introducing a gas into saidmicroscopic voids between said platen and said wafer at a pressure lessthan said preloading contact pressure but above the pressure sufficientto cause bowing and reduced thermal transfer with a wafer which is notpreloaded.
 10. Apparatus for effecting thermal transfer in accordancewith claim 9 wherein said platen has a convexly curved surface. 11.Apparatus for effecting thermal transfer in accordance with claim 10wherein said convexly curved platen surface has a finished metallicsurface for receiving said semiconductor wafer.
 12. Apparatus foreffecting thermal transfer in accordance with claim 11 wherein saidfinished metallic surface is a finished metallic surface of aluminum.13. Apparatus for effecting thermal transfer in accordance with claim 10wherein said convexly curved platen has a resilient, thermallyconductive polymer surface affixed thereto for receiving saidsemiconductor wafer.
 14. Apparatus for effecting thermal transfer inaccordance with claim 9 wherein said means for introducing a gascomprises a channel within said platen which is connected externally toa gas source and terminates in an open circular channel in the surfaceof said platen having a radius slightly less than that of said waferwhich is loaded onto said platen.
 15. Apparatus for effecting thermaltransfer in accordance with claim 14 in combination with sealing meansembedded in the surface of said platen external to said open circularchannel to prevent gas from entering the processing chamber. 16.Apparatus for effecting thermal transfer in accordance with claim 15wherein said platen further includes a second network of channels forcirculating a heat transfer fluid.
 17. Apparatus for effecting thermaltransfer with a semiconductor wafer in a vacuum processing chamber,comprising:a platen having a nonplanar surface for receiving said waferand forming a solid-to-solid contact with the back side of said wafer;means for loading said wafer at its periphery against said platensurface so as to produce a platen-to-wafer preloading contact pressureover the surface area of said wafer sufficient to maintainsolid-to-solid contact between said wafer and said platen when gas isintroduced into the microscopic voids between the platen and the wafer;and means for introducing a gas into said microscopic voids between saidplaten and said wafer at a pressure less than said preloading contactpressure, said means for loading said wafer applying a preloading forceto permit said wafer to accommodate gas pressure across its back side inthe range of 5 Torr to 100 Torr without lifting or bowing of the wafer.